THE MOVEMENTS AND REACTIONS OF AMCEBA. 171 



the anterior edge. The form of the Amoeba shows that this cannot be 

 due to pressure from behind, for if the pressure were greatest behind 

 and less in front, the mass of internal fluid would, of course, be forced 

 forward, and the Amoeba would be thickest in front instead of behind ; 

 the form of anterior and posterior ends would be reversed. In the 

 varied modeling of the anterior end we have seen another proof of the 

 impossibility of accounting for the action here by pressure from behind 

 (p. 164). Further, the forward current on the upper surface of the 

 Amoeba could not possibly be produced by pressure from behind. 

 The impossibility of accounting for the form and movement by pres- 

 sure from behind has been recognized by most investigators, though 

 on other grounds than those here set forth. 



On the other hand, if we take the view that the anterior wave, after 

 attaching itself to the substratum, exercises a pull on the parts behind 

 it, the rest of the phenomena follow most naturally. Such a pull 

 would draw forward the tenacious upper layer of the body, and we 

 find that this is actually moving forward. The mass of inactive inter- 

 nal contents would drag behind as far as possible, so that the thickest 

 part of the Amoeba would be at the rear, and this is exactly what we 

 find to be true. The posterior end would be dragged forward. This, 

 also, is true. In its movement forward it would be partly rolled ; that 

 is, its lower surface would gradually pass upward and become the upper 

 surface. This, also, we know to be true.* Finally, the internal fluid 

 contents would be compelled to stream forward as the anterior end 

 advanced and the posterior end followed it. This streaming is, of 

 course, one of the striking features of the Amoeba. The character- 

 istics of the endosarcal streaming are, I believe, exactly what might 

 be expected from the method of origin just set forth. 



We have one other more or less independent factor of the movement 

 in the contraction of the posterior part of the body that is not in con- 

 tact with the substratum. As we have seen, the substance of the sides 

 and bottom, as well as of the upper surface, are moving forward in 

 this region, as indicated by the small arrows of the diagram (between 

 xandy). Moreover, we know that there is a lateral contraction as 

 well as an antero-posterior one, for the wide, flat anterior portion 

 shrinks together as soon as it is released from the substratum. This 

 contraction should probably be brought into relation with the previous 

 increase of surface at the anterior end. As the anterior wave is sent 

 forth the surface at the anterior end is much increased. We might com- 

 pare the action with the stretching of a sheet of India rubber. This 

 tense portion then becomes attached to the substratum, as we might 



* Large bodies within the Amoeba close against the posterior surface are often 

 rolled over in this process, as I have several times observed. 



